1. Field of the Invention
The present invention relates to a digital communication apparatus such as an automobile telephone, a portable telephone, digital telephone and a cordless telephone which are operated in a digital mode and, more particularly, to a digital communication apparatus capable of detecting an error in a plurality of types of data having different transfer formats with simplified circuit arrangements.
2. Description of the Related Art
A radio telephone system having a digital mode has recently been proposed in which not only control signals but also speech signals are digitally transmitted between a base station and a mobile unit. Since the system is able to secure secrecy, improve in affinity with data, and effectively use a radio frequency, it attracts great attention.
FIG. 5 shows an example such a proposed communication system. This system includes a control station CS connected to a cable telephone network NW, a plurality of base stations BSl to BSn connected to the control station CS through cable lines CLl to CLn, and a plurality of mobile stations PS1 to PS4 to PSm (not shown). The base stations BSl to BSn respectively form radio zones El to En in different areas. The mobile stations PSl to PSm are connected to the base stations BSl to BSn in the same radio zones, and the base stations are connected to the cable telephone network NW through the control station CS.
The communication system generally performs radio communication between the base stations and mobile stations using TDMA (Time Division Multiple Access) system. A plurality of radio frequencies are assigned to each of the mobile stations PSI to PSm. Signals transmitted at the frequencies form a plurality of frames, and each of the frames includes, for example, six slots. When communication is performed by one of the mobile stations PSi (i=1, 2, 3, ..., m) a vacant slot having an arbitrary radio frequency is assigned to the mobile station PSi. Therefore, radio communication is performed between the mobile station PSi and base station BSi using the slot assigned to the mobile station PSi.
FIG. 6 shows a signal transfer format of slots transmitted from the base stations to the mobile stations. According to the format, a 28-bit sync signal SYNC is arranged in the initial field, then 12-bit low-speed data of slow associated control channel (SACCH), 130-bit high-speed data of fast associated control channel of (FACCH), and a 12-bit verification code of coded digital verification color code (CDVCC) are arranged in the next field in this order, and 130-bit high-speed data of FACCH and a 12-bit preliminary bit are arranged in the last two fields. It is the field of FACCH (260 bits=65.times.4) that is used to transmit control data and to convolution-encode four times as much data as original data of 65 bits.
FIG. 7B shows a format of the original data of FACCH. In this format, a 1-bit continuation flag CF is arranged before a 48-bit message, and a 16-bit cyclic redundancy check (CRC) code (as a error detecting code) is arranged after the message.
FIG. 7A shows a format of the original data of SACCH. The SACCH is mainly used to transmit data of calling and is a field for convolution-encoding two times as much data as original data of 66 bits. As shown in FIG. 7A, a 1-bit continuation flag CF and a preliminary bit R are arranged before a 48-bit message, and a 16-bit CRC code is arranged after the 48-bit message, as in the format shown in FIG. 7B.
In the FACCH, one data of 65 bits is transmitted in one slot. In the SACCH, one data (66 bits) is distributed into 11 slots, and a 6-bit data is thus transmitted in one slot. In other words, two data of FACCH and SACCH having different transfer formats are transmitted in each of the slots. To deal with the two different signal formats: formats in SACCH and FACCH, two independent circuit arrangement for checking the CRC codes following the message signals are required. The result is that the number of components needed for the mobile unit is increased, thereby increasing the cost of manufacturing the mobile unit.